Background: Many studies have shown that greenness has beneficial health effects, particularly on
psychological and cardiovascular outcomes.

Objective: In this narrative review, we provide a synthesis of knowledge regarding greenness exposure and respiratory health. 

Methods: The following outcomes were reviewed: respiratory mortality, lung cancer mortality, lung cancer, respiratory hospitalisations, lung function, chronic obstructive pulmonary disease (COPD), and asthma. We identified 174 articles through a literature search in PubMed, of which 42 were eligible
for inclusion in this review.

Results: The most common marker for greenness exposure was the
Normalised Difference Vegetation Index (NDVI), which was used in 29 of 42 papers. Other
markers used were tree canopy cover, landcover/land-use, plant diversity, density of tall
trees and subjectively perceived greenness. We found beneficial effects of greenness in most
studies regarding respiratory mortality, lung cancer incidence, respiratory hospitalisations
and lung function. For lung cancer mortality, the effects of greenness were less clear-cut, and for asthma and COPD both beneficial and harmful effects of greenness exposure was identified. 

Conclusion: Greenness exposure may be beneficial by promoting physical activity, decreasing stress, reducing air pollution and increasing diversity in the human microbiome. However, some aspects of greenness may be harmful, and greenness may have different health effects in different population sub-groups. Future studies of greenness and respiratory diseases should focus on asthma and COPD, on effects in different population sub-groups and on disentangling health effects of the various
greenness dimensions.